Cardiovascular diseases remain a leading cause of morbidity and mortality. Many of these diseases originate at least in part from genetic defects that impact the development and maturation of the cardiovascular system. Despite substantial progress in our understanding of cardiovascular development and its role in disease susceptibility, much remains to be learned. The focus of this proposal is on cardiac trabeculation, a fundamental aspect of cardiac development that modulates cardiomyocyte mass and cardiac function. Interestingly, trabeculae initially form only in the ventricle but not the atrium, and within the ventricle, they only form in the outer but not the inner curvature. These observations indicate that cardiac trabeculation is regulated by genetic as well as epigenetic factors, such as flow. We will continue to take advantage of the zebrafish system to investigate several aspects of cardiac trabeculation. Our data indicate that 1) cardiac trabeculation initiates by the directed migration/delamination of cardiomyocytes from the compact layer, 2) in zebrafish, as in mouse, Erbb2 signaling is absolutely required for cardiac trabeculation (but importantly why or how it is required remains unclear), and 3) cardiac function is required for cardiac trabeculation. These and other data lead us to propose to test the following three hypotheses: 1) cardiac trabeculation initiates via the directed migration/ delamination of cardiomyocytes. We will analyze at single cell resolution a) the distribution and behavior of small clones of labeled cardiomyocytes as trabeculae form, b) the patterns of cardiomyocyte division during the initiation and expansion of trabeculae, and c) apicobasal polarity of cardiomyocytes in the compact and trabecular layers. 2) Erbb2 signaling regulates cardiac trabeculation via its role in modulating cardiomyocyte epithelial to mesenchymal transformation (EMT) including the loss of apicobasal polarity. Experiments are designed to a) test the cell autonomy of Erbb2 function in the initiation of cardiac trabeculation, b) investigate the regulation of Neuregulin (Nrg) in the initiation of cardiac trabeculation, c) investigate the mechanism of action of Erbb2 signaling in the initiation of cardiac trabeculation, and d) investigate the potential link between Erbb2 signaling and cardiomyocyte EMT, loss of apicobasal polarity and apical constriction. 3) cardiomyocyte contractility and shear stress regulate cardiac trabeculation. Experiments are designed to a) test the role of cardiomyocyte contractility in the initiation of cardiac trabeculation, and b) test the role of flow in the initiation of cardiac trabeculation. These studies will help understand mechanisms of cardiac trabeculation. They should also shed new light on the function of Erbb signaling in cardiomyocyte biology beyond trabeculation as Erbb signaling has been implicated in differentiated cardiomyocyte proliferation and repair of heart injury. Furthermore, they should help elucidate the critical, yet poorly understood, role of cardiac function in cardiac development.